Regulation of IS1-mediated transposition in E. coli is being studied. Of 50 compounds tested, acetate appears to enhance transposition of Tn9 (IS1-cat-IS1). UV irradiation of lambda::Tn9 reduces transposability exponentially; however the reduction of IS1-cat-IS1 transposition is less than IS1-lambda-IS1 transposition. The expectation that portions of the E. coli chromosome should behave as IS1 flanked transposons has not borne out by experiment. One possibility is that resident chromosomal IS1s are inactive.